U.S. Secretary of Defense James Mattis met with Prime Minister Shinzo Abe on Feb. 2, and both reaffirmed the importance of the Japan-U.S. alliance while Mattis indicated that the United States' focus on the Asia-Pacific region was unwavering.

During the U.S. presidential election campaign, then-candidate Donald Trump demanded that Japan and South Korea shoulder more of the load for hosting U.S. troops on their territory if they didn't want Washington to pull out its forces. For a time, he also suggested that the U.S. allies acquire nuclear weapons. This talk from the man who is now president had both Japan and South Korea worried about how the U.S. would deal with them. Mattis' first overseas trip to South Korea and Japan was intended to calm those worries, and his judgment was on the mark.

Before arriving in Japan, Mattis held talks with his South Korean counterpart Han Min-koo. There, regarding the threat from North Korea's nuclear weapon and missile programs, Mattis made a firm promise to continue to expand the U.S.'s deterrent capabilities to defend its allies, including the continued protection of the U.S. nuclear umbrella.

At his meeting with Abe, Mattis shared Japan's concerns over tensions in the East and South China seas. Furthermore, he reconfirmed that the Senkaku Islands in Okinawa Prefecture -- also claimed by China -- were covered by the Japan-U.S. security treaty's Article 5, which obliges the U.S. to aid Japan against armed attack.

The United States is vital to stability in the Asia-Pacific. If the Trump administration were to pursue "transactional" diplomacy -- demanding allies pay more for the luxury of American protection -- and threaten to reduce its engagement in the region, that stability would suffer. Surely that would not be in the interests of the U.S., which has so tightly tied its own economic growth to that of Asia. The Japanese government must explain this reality to the U.S. in no uncertain terms.

Regarding the ongoing controversy over the relocation of U.S. Marine Corps Air Station Futenma in Ginowan, Okinawa Prefecture, Abe and Mattis reaffirmed that the move to the Henoko district of Nago, also in Okinawa, was the only viable path. They also apparently agreed that efforts should continue to reduce the U.S. base burden on Japan's southernmost prefecture.

However, considering Trump's past rhetoric, it is difficult to imagine that the U.S. administration will take any new steps to reduce the U.S. military presence in Okinawa. Should Tokyo and Washington go ahead with the base relocation while promising to carry out a mere nominal "base burden reduction," then the rift between the national and Okinawan governments is likely to deepen, which would in turn act against the smooth operation of the Japan-U.S. alliance.

We praise Mattis' visit to Japan and South Korea up to a point, but still have significant worries about future U.S. involvement in the region.

During his current trip, Mattis did not take up the subject of how much Japan pays to cover the cost of hosting U.S. bases. However, we may yet see the Trump administration lean on Japan to cough up more. It is also certainly possible that the new administration will demand Japan increase defense spending and broaden the operational role of the Self-Defense Forces. Japan should not allow itself to be pressured by the U.S., nor should it respond by boosting defense outlays.

Trump trusts Mattis, and appears to respect the former Marine general's judgment to some extent. However, it's certainly possible that Trump will make decisions linking economic and security issues.

The South Korean political situation is currently in flux, meaning Japan's regional security role has become all the more important. We hope that the Abe-Trump summit set for Feb. 10 will contribute to the continuing stability of the Asia-Pacific region.